Redcay, in U.S. Pat. No. 2,786,626 discloses direct injection of cooling liquid into the gases undergoing compression. There is no interstage cooling since the coolant is sprayed directly into the compressor. Water or low boiling hydrocarbon or both may be used but in the latter case the water and hydrocarbon are employed as a mixture.
A common interstage cooling technique is to use a shell-and-tube heat exchanger and a drum to separate liquid from gas.
An article in the Oil and Gas Journal, Apr. 2, l979, p. 74, discloses water as the cooling medium.
Schuster in U.S. Pat. No. 3,947,146 carries out multistage compression with interstage cooling of hydrocarbon process gas by a direct contact cooling system in cooling towers. In this scheme the hydrocarbon coolant is not kept separate from the water coolant. The patent shows that some of the hydrocarbons condense out of the process gas in each stage and remain with the water as a supplementary cooling medium in the cooling tower. However, it has been found that there are high compressor power requirements associated with this scheme. The hydrocarbon is permitted to run down the cooling tower, with the water, where it is to be skimmed off the water at the bottom. This has the disadvantage of causing a large part of the hydrocarbon to be stripped out overhead without cooling the compressed gas, which in turn causes buildup of a large hydrocarbon recycle that increases the load on the compressor. The stripping of the hydrocarbon overhead results in evaporative cooling of the water so that the temperature of the water at the bottom is almost the same as at the inlet to the tower (it should be 20.degree. to 30.degree. C. higher) whereas the compressed gas leaving the top of the tower is 5.degree. to 10.degree. C. higher than would be expected. The presence of the hydrocarbon in the water to the tower and its subsequent vaporization prevent the water from cooling the gas. In effect, this cools the water instead of cooling the gas. The high hydrocarbon recycle rate and the high inlet temperature of the gas to the subsequent compression stage, result in high compressor power requirements.
According to the present invention, this malfunctioning is corrected by separating water and hydrocarbon in the cooling tower, as described in the following.